The sets are: 1x3, 2x3, 1x4, 2x4, 1x5, 2x5, 1x5, 2x4, 1x4, 2x3, 1x3 - they can be made up of the same card or a run from a suit - the name Twos refers to the 2s being the joker, which can therefore replace any card or for instance to make a 5 from cards of the same value obviously only 4 in a pack the 2 makes it 5.
The aim for each round is to match a set and then to get as many more sets as possible to get the highest score. My family play Twos a lot - so much so that my parents have a running tournament between the 2 of them alternating with Chinese Checkers: It's a version of Rummy each player is dealt 10 cards and the remaining is placed in the centre with the top card in the well face up, or the player to the left of the dealer is dealt 11 cards and throws one card into the well.